1. Field of the Invention
The present invention relates to an air supplying structure for multi-cylinder engine, and more particularly to a multi-cylinder engine including an 18-cylinder engine body to be supercharged in a two-staged manner.
2. Description of Related Art
In vehicular engines, it has been known to provide superchargers for pressurizing intake air and feeding it into cylinders. Such superchargers are classified into: a mechanically driven type for compressing the intake air by the driving force of the engine itself; and an exhaust-gas turbine type for compressing the intake air by rotating a turbine by the exhaust gas of the engine.
Exhaust-gas turbine type superchargers are provided with a turbine, a compressor, and a shaft member for coupling the turbine and compressor, such that the turbine is rotated by an exhaust gas to thereby rotate the compressor through the shaft member, so as to compress the intake air by the compressor.
Differently from the exhaust-gas turbine type superchargers, the shaft member is rotated by mechanical means such as a motor in mechanically driven type superchargers, so as to rotate the compressor to thereby compress the intake air.
In engines having such superchargers, compressing the intake air by the supercharger elevates the temperature of the intake air to thereby eventually decrease the density of the intake air, thereby possibly increasing a heat load such as onto the cylinders and/or lowering the supercharging efficiency. Particularly, in an air-supplying structure for conducting a multistage supercharge by series connecting superchargers so as to stepwise supercharge, the temperature of the intake air is further elevated, thereby considerably lowering the supercharging efficiency and increasing the heat load onto the cylinders.
To solve such problems, it has been known to provide a cooler for cooling the compressed intake air, in air-supplying structures of engines. For example, in an air-supplying structure for conducting the two-staged supercharging, there are provided: an inter-cooler between series connected two superchargers; and an after cooler between the downstream side supercharger and the cylinders so as to cool the intake air compressed by and supplied from the upstream side and downstream side superchargers, respectively.
Meanwhile, in a multi-cylinder engine adopting the two-staged supercharging scheme, such as in a V-type 12-cylinder engine including totally 12 cylinders comprising two 6-cylinder banks, there are totally provided four pairs of superchargers such that each three adjacent cylinder set of one bank is connected to one pair of superchargers which cooperatively conduct the two-staged supercharging. In such a 12-cylinder engine, there are provided two intercoolers for cooling the compressed intake air, one at the front and the other at the back of the engine, such that the intercooler provided at the front is connected with the forwardly provided two pairs of superchargers and the intercooler provided at the rear is connected with the rearwardly provided two pairs of superchargers. Alternatively, the two pairs of superchargers connected to one bank may be connected to the (forward) one of the intercoolers, and the other two pairs of superchargers connected to the other bank may be connected to the (rearward) other of the intercoolers.
Further, such as in a V-type 16-cylinder engine including totally 16 cylinders comprising two 8-cylinder banks, there are totally provided four pairs of superchargers such that each four adjacent cylinder set of one bank is connected to one pair of superchargers which cooperatively conduct the two-staged supercharging. Here, the size of the 16-cylinder engine is larger than that of the 12-cylinder engine, to the extent of the increased number of cylinders. In such a 16-cylinder engine, there are provided two intercoolers similarly to the 12-cylinder engine, one at the front and the other at the back of the engine, such that the intercooler provided at the front is connected with the forwardly provided two pairs of superchargers and the intercooler provided at the rear is connected with the rearwardly provided two pairs of superchargers.
As described above, those engines having a lesser number of cylinders are constituted such that all of the superchargers connected to the cylinders of one bank are connected to one of the two intercoolers, while those engines having an even number of cylinders provided at each bank are constituted such that the cylinders are divided into the front and rear groups and the front side superchargers connected to the front group cylinders are connected to the front intercooler and the rear side superchargers connected to the rear group cylinders are connected to the rear intercooler.
Meanwhile, in large-sized vehicles such as construction machines, there has been demanded a larger-sized engine due to the increasing working loads. However, such a larger-sized engine is to have a larger number of cylinders, to thereby cause such a problem that air-supplying structures of a good efficiency in no way are obtained such as by the provision of superchargers in the same manner as the conventional.
For example, in an engine including totally 18 cylinders comprising one 9-cylinder bank and the other 9-cylinder bank, connecting all of the superchargers of one of the banks to one of the inter-coolers will increase the length of tubes between the superchargers and inter-coolers due to the increased number of cylinders.
Further, dividing the cylinders into two groups in the fore-and-aft direction leads to groups of 4 cylinders and 5 cylinders, because one bank is provided with an even number of cylinders (i.e., nine pieces of cylinders). This requires preparation of two types of volumes (sizes) of superchargers including one to be connected to 5 cylinders and the other to be connected to 4 cylinders, to thereby problematically increase the types of constituent parts as well as the cost.
The present invention has been carried out in view of the problems as described above, and it is therefore an object of the present invention to provide an air supplying structure for a multi-cylinder engine capable of commonizing constituent parts and of shortening the tubes connecting between superchargers and coolers.
Namely, the present invention provides an air supplying structure for a multi-cylinder engine for conducting a two-staged supercharging to an engine body totally including 18 pieces of cylinders comprising nine pieces of cylinders serially arranged in one bank and nine pieces of cylinders serially arranged in the other bank, the air supplying structure for multi-cylinder engine comprising: totally six sets of supercharging units, each set of supercharging unit being constituted of a low-pressure stage supercharger and a high-pressure stage supercharger which are series arranged with each other so as to conduct the two-staged supercharge; each of the high-pressure stage superchargers including a compressor having an outlet connected to three pieces of adjacent cylinders which are provided in the same bank of the engine body; totally six pieces of aftercoolers, each aftercooler being connected to an intake air passage between each of the high-pressure stage superchargers and the three pieces of cylinders; and totally two pieces of intercoolers, each intercooler being connected to those outlets of compressors of the three pieces of the low-pressure stage superchargers; wherein the two pieces of inter-coolers are arranged at the opposite sides of the engine body, respectively, along the arranging direction of the cylinders; wherein the three pieces of the low-pressure stage superchargers to be connected to one intercooler of the two pieces of intercoolers are connected, through the high-pressure stage superchargers series arranged to the three pieces of low-pressure stage superchargers, to the three pieces of cylinders in the one bank and the six pieces of cylinders in the other bank, which cylinders are arranged near the one intercooler; and wherein the three pieces of the low-pressure stage superchargers to be connected to the other intercooler of the two pieces of intercoolers are connected, through the high-pressure stage superchargers series arranged to the three pieces of low-pressure stage superchargers, to the six pieces of cylinders in the one bank and the three pieces of cylinders in the other bank, which cylinders are arranged near the other intercooler.
According to the present invention, each intercooler and each aftercooler are provided at the outlet sides of the associated low-pressure stage supercharger and high-pressure stage supercharging, respectively, in the air-supplying structure for conducting the two-staged supercharging by the 6 sets of series arranged low-pressure stage supercharger and high-pressure stage supercharger, thereby allowing to highly compress the intake air and to lower the temperature of the intake air. This allows an increase in the amount of the intake air for each cylinder to thereby increase the output power of the engine.
Further, each aftercooler is connected to three pieces of cylinders in the 18-cylinder engine thereby allowing to downsize the aftercoolers. Since the cylinders are divided into 3-piece groups of cylinders and each three pieces of cylinders are connected to each associated aftercooler, all of the six pieces of aftercoolers can be standardized to a 3-cylinder size.
Moreover, since each high-pressure stage supercharger is operatively connected to three pieces of cylinders, the volumes of the six pieces of high-pressure stage superchargers can be standardized to a smaller or 3-cylinder size, and the volumes of the six pieces of low-pressure stage superchargers series arranged to the six pieces of high-pressure stage superchargers, respectively, can be also standardized to a smaller or 3-cylinder size. This allows to commonize the constituent parts, thereby reducing the manufacturing cost.
Each intercooler is connected with totally three pieces of low-pressure stage superchargers comprising: one piece of low-pressure stage supercharger connected to the three pieces of cylinders in one bank near the intercooler; and two pieces of low-pressure stage superchargers connected to the six pieces of cylinders in the other bank near the intercooler. This allows to shorten the length of tubes for connecting these low-pressure stage superchargers and the associated intercooler, than the situation where all of the low-pressure stage superchargers connected to the cylinders of one bank are connected the one intercooler. Further, by connecting the remaining three pieces of low-pressure stage superchargers to the other intercooler, the tubing constitution for connecting the two pieces of intercoolers and six pieces of low-pressure stage superchargers can be constituted in a centrosymmetric manner about the substantial center of the engine, to thereby commonize the tubing parts at the forward side and rear side of the engine.
Also, the tubing constitution for connecting the six pieces of after-coolers and six pieces of high-pressure stage superchargers are constituted in a centrosymmetric manner about the substantial center of the engine similarly to the aforementioned tubing constitution for connecting the intercoolers and low-pressure stage superchargers, thereby allowing to commonize the tubing parts.
In the present invention, the intercoolers and the aftercoolers are preferably supplied with a cooling liquid introduced from a cooling circuit separated from an engine body cooling circuit for cooling the engine body.
Usually, temperatures of a cooling liquid for cooling an engine body such as an engine block are set near 80xc2x0 C. so as to maintain a combustion efficiency. As such, the temperature of the intake air can not be sufficiently lowered, when the cooling liquid from the engine body cooling circuit for cooling the engine body is introduced into the intercoolers and aftercoolers, for example.
In the present invention, the intercoolers and the aftercoolers are supplied with a cooling liquid introduced from the cooling circuit separated from the engine body cooling circuit, so that the cooling liquid at sufficiently lower temperatures can be supplied to the intercoolers and aftercoolers, to thereby sufficiently lower the temperature of the intake air. This allows to increase the density of the intake air to thereby increase the amount of the intake air into the cylinders, thereby assuredly increasing the output power of the engine.
Further, the intake air compressed by the low-pressure stage supercharger can be cooled by the intercooler having the lower coolant temperature, so as to increase the density of the intake air compressed by the low-pressure stage supercharger, to thereby allow to decrease the flow rate per unit weight at the inlet of the high-pressure stage supercharger. This allows to downsize the high-pressure stage supercharger, to thereby improve the rapid acceleration responsibility of the engine.